Groping in the fog: Soaring migrants exhibit wider scatter in flight directions and respond differently to wind under low visibility conditions

Atmospheric conditions are known to affect flight propensity, behaviour
during flight, and migration route in birds. Yet, the effects of fog have
only rarely been studied although they could disrupt orientation and
hamper flight. Fog could limit the visibility of migrating birds such that
they might not be able to detect landmarks that guide them during their
journey. Soaring migrants modulate their flight speed and direction in
relation to the wind vector to optimise the cost of transport.
Consequently, landmark-based orientation, as well as adjustments of flight
speed and direction in relation to wind conditions, could be jeopardised
when flying in fog. Using a radar system operated in a migration
bottleneck (Strait of Messina, Italy), we studied the behaviour of soaring
birds under variable wind and fog conditions over two consecutive springs
(2016 and 2017), discovering that migrating birds exhibited a wider
scatter of flight directions and responded differently to wind under fog
conditions. Birds flying through fog deviated more from the mean migration
direction and increased their speed with increasing crosswinds. In
addition, airspeed and groundspeed increased in the direction of the
crosswind, causing the individuals to drift laterally. Our findings
represent the first quantitative empirical evidence of flight behaviour
changes when birds migrate through fog and explain why low visibility
conditions could risk their migration journey.